the lipid A, such that laurate is replaced by the unsaturated fatty acid palmitoleate in
LPS so as to readjust the outer membrane fluidity after cold shock (Carty et al. 1999).
During fractionation of the subcellular components of Antarctic bacteria Micrococcus roseus and Sphingobacterium antarcticus, carotenoids were always found to
be associated with membranes. Therefore, a role of the carotenoids in the regulation
of membrane fluidity was postulated. A trend of increase in the amount of non-polar
carotenoids was also observed in both the organisms when they were grown at low
temperature, compared to ambient temperature (Chattopadhyay and Jogannadham
2001). These researchers also found an increasing trend in the amount of polar
carotenoids and decrease in the amount of non-polar carotenoid in both the
organisms when they were grown at low temperature compared to their production
profile obtained by growth at room temperature. Therefore, their finding suggested
that in response to the increase in the synthesis of membrane-fluidizing fatty acids,
synthesis of membrane-rigidifying polar carotenoids was also enhanced to counterbalance the effects of fatty acids in the Antarctic bacteria (Chattopadhyay and
Jagannadham 2001).
Denarie et al. (1992) emphasized that maintenance of membrane fluidity is a
major mode of survival of cold-adapted rhizobia since the symbiotic proteins (p Sym
Nod) which are major determinants of nodule competitiveness are membrane
associated. However, as per findings of Geiger et al. (1993) the induction of nod
FE gene in cold-adapted R. leguminosarum bv. viciae was found to result in the de
novo synthesis of phospholipids with specific polyunsaturated fatty acids. Drouin
et al. (2000) observed that low temperature conditions affected fatty acid composition of all rhizobial strains, regardless of their cold adaptation level. The proportion
of unsaturated fatty acids also increased significantly with the decrease in the growth
temperature from 25 to 5
C. A specific fatty acid (cis-12 octadecenoic acid) was
detected in arctic rhizobial strains during growth at 5
C.
6.7
Cold-Active Enzymes
The most important selective pressure of low temperatures is exerted on chemical
reaction rates causing it to drop exponentially. Enzymes are biological catalyst and
involved in most of the chemical reactions in the cell, those are necessary for the cell
survival. Adaptation of the cells to the low temperature requires the presence of
intracellular enzymes, which are active at low temperature. These cold-active
enzymes have high catalytic efficiency (K cat /K m ) at low and moderate temperatures
(0–30
C) at which homologous enzymes produced by microorganism from other
thermal classes are poorly active or not active at all. In addition, these enzymes are
generally thermolabile; their activity is shifted toward low temperature (Margesin
et al. 2007). The commonly accepted hypothesis for this cold adaptation is the
activity–stability–flexibility relationship, which suggests that psychrophilic
enzymes increase the flexibility of their structure to cope freezing effect of cold
habitats (Somero 2004).
6 Plant Growth Promoting Rhizobacteria: Mechanisms and Alleviation of Cold Stress. . . 195
LPS so as to readjust the outer membrane fluidity after cold shock (Carty et al. 1999).
During fractionation of the subcellular components of Antarctic bacteria Micrococcus roseus and Sphingobacterium antarcticus, carotenoids were always found to
be associated with membranes. Therefore, a role of the carotenoids in the regulation
of membrane fluidity was postulated. A trend of increase in the amount of non-polar
carotenoids was also observed in both the organisms when they were grown at low
temperature, compared to ambient temperature (Chattopadhyay and Jogannadham
2001). These researchers also found an increasing trend in the amount of polar
carotenoids and decrease in the amount of non-polar carotenoid in both the
organisms when they were grown at low temperature compared to their production
profile obtained by growth at room temperature. Therefore, their finding suggested
that in response to the increase in the synthesis of membrane-fluidizing fatty acids,
synthesis of membrane-rigidifying polar carotenoids was also enhanced to counterbalance the effects of fatty acids in the Antarctic bacteria (Chattopadhyay and
Jagannadham 2001).
Denarie et al. (1992) emphasized that maintenance of membrane fluidity is a
major mode of survival of cold-adapted rhizobia since the symbiotic proteins (p Sym
Nod) which are major determinants of nodule competitiveness are membrane
associated. However, as per findings of Geiger et al. (1993) the induction of nod
FE gene in cold-adapted R. leguminosarum bv. viciae was found to result in the de
novo synthesis of phospholipids with specific polyunsaturated fatty acids. Drouin
et al. (2000) observed that low temperature conditions affected fatty acid composition of all rhizobial strains, regardless of their cold adaptation level. The proportion
of unsaturated fatty acids also increased significantly with the decrease in the growth
temperature from 25 to 5
C. A specific fatty acid (cis-12 octadecenoic acid) was
detected in arctic rhizobial strains during growth at 5
C.
6.7
Cold-Active Enzymes
The most important selective pressure of low temperatures is exerted on chemical
reaction rates causing it to drop exponentially. Enzymes are biological catalyst and
involved in most of the chemical reactions in the cell, those are necessary for the cell
survival. Adaptation of the cells to the low temperature requires the presence of
intracellular enzymes, which are active at low temperature. These cold-active
enzymes have high catalytic efficiency (K cat /K m ) at low and moderate temperatures
(0–30
C) at which homologous enzymes produced by microorganism from other
thermal classes are poorly active or not active at all. In addition, these enzymes are
generally thermolabile; their activity is shifted toward low temperature (Margesin
et al. 2007). The commonly accepted hypothesis for this cold adaptation is the
activity–stability–flexibility relationship, which suggests that psychrophilic
enzymes increase the flexibility of their structure to cope freezing effect of cold
habitats (Somero 2004).
6 Plant Growth Promoting Rhizobacteria: Mechanisms and Alleviation of Cold Stress. . . 195